This invention relates in general to radar systems and especially to radar systems of the moving-target indication-type (MTI). More particularly, the present invention relates to arrangements which compensate for relative motion between fixed objects and an airborne radar system to provide improved cancellation of the signals from the fixed objects.
A coherent MTI radar system uses the phase characteristic present in the backscattered radar pulse to distinguish between the returns from moving targets and those from stationary objects or clutter. When the radar is fixed in space, the phase relationship existing at the radar receiver between the transmitted pulses and the backscattered pulses from a fixed object is the same for successive pulses whereas the phase relationship of the backscattered pulses from a moving target is continually changing. This variation in relative phase between the transmitted pulses and their corresponding returns from a relatively moving target, as contrasted with the unchanging phase of returns from a relatively fixed target, provides a means of distinguishing between stationary and moving targets. However, in an airborne moving-target-indication (AMTI) radar system, the clutter sources move relative to the radar platform so that the relative phase of the backscattered signals originating from them varies from pulse to pulse.
The relative displacement of the radar antenna with respect to a particular clutter scatterer depends on the velocity of the aircraft and the direction of the scatterer relative to the velocity vector of the aircraft. In general, the velocity of the aircraft has a component that is in the direction that the antenna is pointing (parallel to the antenna's boresight direction) and a component that is normal to this direction. Each component produces different effects which complicate clutter cancellation. The velocity component parallel to the antenna boresight direction primarily causes the center frequency of the clutter to vary with range and azimuth, while the velocity component normal to the antenna's boresight direction causes the clutter power spectrum to be spread as a function of the aircraft velocity and the antenna's beamwidth. When the antenna is pointing fore or aft, the variation of the centroid of the clutter predominates; when the antenna is pointing abeam, the velocity spread across the antenna's beamwidth is more important. At intermediate pointing angles, both spreading and variation of the centroid occur due to the aircraft's motion.
In the past, one method of compensating for the motion normal to the antenna's boresight direction has been to use antenna systems in which the antenna's phase center may be physically or electronically displaced along a plane that is normal to the antenna's boresight direction. The phase center of an antenna beam is the point about which rotation of the pattern will produce the minimum change in the far field pattern or, in effect, the point in space from which the beam appears to emanate. By this technique, known as displaced phase center antenna (DPCA), one radiation pattern, corresponding with one transmission-reception cycle, effectively occupies the same position in space as did another pattern an interpulse period before so that the component of motion normal to the boresight direction is effectively cancelled out as far as returns from fixed objects are concerned. U.S. Pat. No. 3,438,030 by Frank R. Dickey, Jr. and U.S. Pat. No. 3,806,924 by Sidney P. Applebaum are examples of motion compensation arrangements employing antenna systems (a monopulse horn antenna in the former and a linear broadside array antenna in the latter) in which the phase center may be shifted in the plane normal to the antenna's boresight direction. In this type of system, the component of motion parallel to the antenna's boresight direction is dealt with by offsetting the frequency of the return signal by an amount equal to the average doppler frequency of the clutter spectrum. Because the center frequency of the clutter varies with range as well as azimuth and the open- or closed-loop techniques used to determine the doppler-offset frequency are too slow to allow correction for all ranges in a particular sweep, it has been the practice to determine the doppler shift at one particular range and use this value for all ranges. This results in a peak error which has been a prime limitation in the performance of this type of system.
In another approach to the motion compensation problem, the aperture of an array antenna is aligned along the ground track so that the component of motion parallel to the boresight direction is eliminated. The elements of the array are grouped so as to create two antennas displaced by an amount close to the nominal interpulse movement. This displacement is then made exact by varying the pulse repetition frequency so that the interpulse movement corresponds with the antenna separation. This has the advantage of giving a wide angle correction but has the disadvantage of requiring the pulse repetition frequency to be set by the antenna platform parameters. Since the antenna does not rotate through 360.degree., this arrangement provides no coverage in the direction of the aircraft's motion.